'X' is probably just another example of the hyper-marketing of the term derived from Douglas Coupland's book 'Generation X'. Marketing people know that people, in general, associate X with the idea that they are not part of the rat race and all the related materialist, social climbing attitudes.

It short, it means people will buy it if we put an 'X' in front of the name.

Maybe they are trying to project the mysterious. X is the symbol of a unknown in math and when they named the x-ray it was because they didn't know what it was. So maybe they are saying the boat has unknown powers.